Is this already what you have and you are interested in joining the two insulation layers and the airtightness or is this just a possible approach? If you have complete freedom my solution is to put the insulation above the rafters, cut off the ends at the wall plate and join it directly to the EWI. I will then suspend my eaves off an oversized counterbatton. As you have it there is obviously a large thermal bridge between the two insulation layers and it is at a point in the house that heat will naturally rise out of. You could put some IWI on the top of that internal wall and put a triangle of eps in the gap but you are limited in your need for ventilation with that detail. Where is your air tightness layer? Is it internal plaster?

Could you use a second eaves ventilator? This type http://www.cavitytrays.co.uk/ventilation/view/2/81/1/type-oevwf-open-eaves-ventilator. This will leave you with the thinnest insulation at the wall plate zone. If this still bothers you create an insulated skeiling between the flat ceiling and the wall

Posted By: ShevekThe problem with standard ventilators like that is that they require fitting from above the rafters. Is it possible to take just the lower first few runs of roof tiles off?
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Hi , did this yesterday to a place . easy with a tower ,

take the first 3 rows of tiles off , tuck quilt under rafter and against wall at soffit and to meet up with loft quilt.
Cut PUR 800mm long to fit inbetween rafters , push in so as bottom of PUR joins up with EWI or just over wall
if your EWI in the future . replace with breather felt/batton etc , stick 10mm nail-on vents along top of fascia

I've left a 25mm gap above PUR , large rafter 75mm gap at plate junction, so used 50mm pur

slightly different to you as you converting the loft , but with the tile off you can join up the VB and connect
ewi and internal loft pitch insulation

on my own place i used 50mm and have no vent gap as im not concerned re. condensation over this short length as I've a good VB for wall to ceiling connection and cold vented loft

http://www.screwfix.com/p/over-fascia-ventilator-10mm-x-1m-pack-of-10/65515

nail on over fascia vents

Yep like that

As far as i'm aware B.regs require 25mm cont. air gap , most leave 50mm between rafter with old style felt to make up for rafters ?? you could argue with the correct breather felt no need for any airgap, though most require 25mm.

the 10mm height increase wont show in anyway relative to ajoining terrace , just a slight lift in the tile at either end. unnoticable from the ground.

PUR/PIR it's all the same to me , celotex,xtratherm,kingspan
you could use aerogel as it's a small gap you may have to work with. never used it , not sure of ease of supply.
how about phenolic

Lower joist to increase loft space is quiet common . do this first, fit joists, tack out ceilings
then work, doing rip out, from above via scaffold/roofwindow (or loft hatch) , drop in stairs last and skim ceiling over when completed loft . bit of a mess but keeps the first floor liveable

2250mm still feels ok , that would gve you 180mm . I presume you got 75-100mm ceiling joists currently.

I sure you're aware best to use minium height joist possible to maximise hieght saving , perhaps by spanning off mid way supporting wall to plate will reduce span width , rather than sticking in steels like they seem keen to do all the time.
how about I beams , this is interesting
http://www.strongtie.co.uk/iloft/index.php
you could slide in beams from outside , stick them on lower course ,tie them in with rafters , then remove
all unnecessary brickwork /wallplate above this

shame no central supporting wall . I beams probably going to be too deep at 6.2m span
bit of span spec here for I beams
http://www.trussform.co.uk/download/TJI_Technical_Guide.pdf

Since the discussion above we've decided to aim for Passivhaus, which means the above isn't going to cut the mustard. Any ideas?

I'm wondering whether a re-roof is unavoidable. Rip roof and battens up. Add 82.5 mm Kooltherm 18 below rafters, 100 mm K7 full-fill between rafters, breather membrane, new counter-battens (size?), battens, tiles.

This will mean we're higher than the neighbour's roofs. Is this possible?

Okay, this is a preliminary sketch of the eaves detail with new loft conversion above and new insulated skeiling added.

Any thoughts on avoiding interstitial condensation? Do you think that brickwork upstand will stay warm enough?

Air-tightness barrier will be the plaster to walls and plasterboard to skeiling. I could use foil-backed plasterboard for the skeiling but do you think a vapour control layer will make much of a difference here when the existing walls and ceiling don't have one?

The over fascia vent should go on top of the sarking membrane/eaves sheet so that it ventilates the batten cavity as in a warm roof.

If you move the fascia up so that the over fascia vent correctly supports the underside of the tiles then there'll be nothing to fix it to. I would run a continuous tilting fillet or inverted bevelled batten behind the fascia screw fixed to the top side of the rafters.

You probably don't need the short section of soffit board. Just use a fair-faced batten.

David